1. Field of the Invention
The present invention relates to a semiconductor device.
2. Background of the Related Art
Semiconductor devices can include many different forms. One form allows electricity to be transformed into light by applying the light to a luminescent material.
Referring to FIG. 1, a related art semiconductor device is shown with a first electrode 2 formed 20 on a transparent substrate 1, a hole injecting layer (HIL) 3 and a hole transporting layer (HTL) 4 formed on the first electrode 2, a luminescent layer 5 formed on the HTL 4, an electron transporting layer (ETL) 6 and an electron injecting layer (EIL) 7 formed on the luminescent layer 5, and a second electrode 8 formed on the EIL 7. Any one or more of HIL3, HTL 4, ETL 6 and EIL 7 may be omitted, depending on the particular device structure adopted.
Electrons and holes injected into the luminescent layer through the second electrode 8 and the first electrode 2, respectively, recombine to decay radiatively. For most semiconductor devices, the charge injection barrier is higher for electrons than for holes. It is well known that the electron injection barrier may be lowered by employing a low work function material for the second electrode 8. However, low work function materials are chemically reactive, which makes it difficult to use such materials for electrodes. Accordingly, such materials are often used as a second electrode after being alloyed with one of more stable materials, as seen in the examples of Mg:Ag and Al:Li. However, such alloyed second electrodes are still less stable, more costly to form, and more difficult to deposit in a uniform film as compared to aluminum.
An even more serious problem often encountered with an alloyed second electrode of Mg:Ag or Al:Li is the frequent occurrence of cross talk or current leakage between pixels, which may be attributed to the diffusion of Mg or Li ions across organic layers of the device. This problem can be greatly alleviated if one selects aluminum as a second electrode material. However, in the case of aluminum there is a need to improve its poor electron injecting capability. The electron injecting capability of a high work function second electrode, such as aluminum, can be significantly enhanced by inserting a very thin layer (typically 0.3 nm to 1.0 nm) of an electrically insulating material such as LiF, MgF2 or Li2O, inserted either between an aluminum electrode and the luminescent layer, or between the aluminum electrode and the ETL (see, for example, IEEE Transactions on Electronic Devices, Vol. 44, No. 8, p 1245-1248 (1997), the contents of which are incorporated herein in their entirety).
Li2O is a particularly interesting material, in this regard, in that it is an electrically insulating material with a very low work function. The work function of alkali metals themselves is very low, and it becomes even lower when oxidized: for example, work function decreases from 2.1 eV for Cs to about 1 eV for Cs2O. Various alkali metal compounds have reportedly been used to form an insulating buffer layer for the purpose of lowering the electron injecting barrier: e.g., Li2O, LiBO2, NaCl, KCl, K2SiO3, RbCl, and Cs2O to name a few.
Despite this improvement, the introduction of the insulating buffer layer poses a challenging new problem, namely, deterioration of adhesion between an EL multilayer and aluminum, with consequent reduction of life time of the device. Experimental results reveal evidence of poor adhesion either at the buffer layer/aluminum interface or at the EL multilayer/buffer layer interface. This situation is not unexpected, given the different characteristics of materials involved. In summary, semiconductor devices of the related art have at least two basic drawbacks, namely, poor adhesion and short life time.
The above information is expressly incorporated by reference in its entirety herein where appropriate for appropriate teachings of compositions useful in the invention, as well as additional or alternative details, features and/or technical background.